1. Field of the Invention
The present invention relates generally to four wheel drive type vehicles and more specifically to a transfer unit for use in a four wheel drive arrangement which transfer unit permits the torque distribution between the forward and rear wheels to be selectively varied.
2. Description of the Prior Art
FIGS. 1 and 2 show previously proposed transfer gear arrangements for use in four wheel drive vehicles.
In the arrangement shown in FIG. 1 the transfer unit inludes an input shaft 1 which is operatively connected to the engine such as through a clutch and transmission. The input shaft 1 is connected to a forward wheel output shaft 3 by way of a planetary gear unit 2. The planetary gear unit in this instance includes a carrier 2a which is directly connected with the input shaft 1 for synchronous rotation therewith.
The planetary gear further includes a sun gear 2b which is directly connected with the forward wheel output shaft 3 and a ring gear 2c which is connected with a rear wheel output shaft 4 by way of a viscous coupling 5. The input shaft 6 of a differential gear (not comletely shown) is operatively connected with the forward wheel output shaft by way of a drive belt 7.
With this arrangement, the input shaft 1 is operatively connected through the planetary gear unit 2 with rear wheel output shaft 4 and through the planetary gear unit 2 and the viscous coupling 5 to the front wheel output shaft 3. This enables a limited slipe differential gear effect to be established between the forward and rear wheel input shafts 3, 4.
In this arrangement the slip limiting effect is provided by the viscous coupling.
The arrangement shown in FIG. 2 is such that the planetary gear 2 of the above described arrangement is omitted and the input shaft 1 is directly connected to the rear wheel output shaft 4 and the drive connection between the input shaft 1 and the front wheel output shaft 3 established by the viscous coupling alone.
With this latter mentioned arrangement while there is no center differential change effect provided between the input shaft 1 and the front and rear wheel output shafts 3 and 4, the amout of output directed to the front wheel output shaft is limited by the effect of the viscous coupling 5.
However, both of these arrangements have suffered from the drawbacks that the differential effect between the front and rear wheels cannot be removed and the output distribution between the front and rear wheels is specified by the construction of the transfer unit.
Therefore it is not possible with the above arrangements change the mode in which the engine output is distributed in accordance with the surface conditions of the road upon which the vehicle is travelling.
In order to overcome these problems it was proposed in JP-A No. 61-38226 to utilize a multi-plate clutch controlled by a device which allows a controlled amount of slip to occur. However, this arrangement suffered from the problems that the slip control arrangement was expensive and unduly increased the complexity of the arrangement.